Every insect on Earth is an arthropod. Insects belong to the phylum Arthropoda, the enormous animal group defined by jointed legs, segmented bodies, and a hard external skeleton. But the relationship runs only one direction: while all insects are arthropods, arthropods also include spiders, crabs, scorpions, centipedes, and many other creatures that are decidedly not insects. Understanding where insects sit inside this much larger family tree clears up some persistent confusion about which creepy-crawlies are related to which.
What Makes an Animal an Arthropod
Arthropods share a few defining traits that unite the entire phylum, from a housefly to a lobster. The most obvious is the exoskeleton, a rigid outer covering made largely of chitin that serves as both armor and structural support. Because this shell cannot grow with the animal inside it, all arthropods must periodically shed it in a process called molting. That molting cycle is triggered by steroid hormones called ecdysteroids, and the process is so fundamental that it has been conserved across every major arthropod lineage for hundreds of millions of years.1Integrative and Comparative Biology. Evolution of Ecdysis and Metamorphosis in Arthropods: The Rise of Regulation of Juvenile Hormone Jointed appendages give the phylum its name (arthron = joint, pous = foot in Greek), and a segmented body plan rounds out the core package.
Beyond those basics, arthropods display a staggering range of body architectures. Some have two main body regions, some have three, some have dozens of nearly identical segments. Leg counts range from six in insects to over 700 in certain millipedes. Yet all of them trace back to the same ancestral body plan, and the exoskeleton-plus-molting combination is the thread that ties them together. Other proteins woven into that exoskeleton also span the phylum: resilin, a rubber-like elastic protein first discovered in locust wing hinges, has since been found in crustaceans, scorpions, and centipedes as well.2Beilstein Journal of Nanotechnology. Functional diversity of resilin in Arthropoda
The Four Major Arthropod Groups
Modern classification divides living arthropods into four major lineages: chelicerates, myriapods, crustaceans, and hexapods. Chelicerates include spiders, scorpions, ticks, mites, and horseshoe crabs. Myriapods are the centipedes and millipedes. Crustaceans cover shrimp, crabs, lobsters, barnacles, and a long list of less familiar animals. And hexapods are the six-legged group that contains all insects along with a few non-insect relatives.
There is a catch in how these groups relate to one another, and it reshapes the old textbook picture. Molecular studies over the past two decades have shown that crustaceans and hexapods form a single evolutionary clade, now called Pancrustacea. One lineage of crustaceans, the remipedes, appears to be the closest living relative of hexapods. That means the traditional grouping of “crustaceans” as a self-contained unit separate from insects no longer holds up. As one major review put it, it is no longer acceptable to divide arthropods into chelicerates, myriapods, crustaceans, and insects as four parallel branches, because crustaceans are not a neat self-contained group and insects are just one branch within the larger hexapod-crustacean tree.3Current Biology. Arthropod Phylogeny and Evolution
Insects Are Terrestrial Crustaceans
If you have ever looked at a shrimp and thought it vaguely resembled a large insect, your instinct was closer to the science than you might have guessed. Phylogenetic analyses of nuclear genes across dozens of arthropod species place hexapods most closely with branchiopods (fairy shrimp, water fleas) and with two obscure crustacean groups, cephalocarids and remipedes. The conclusion is striking: hexapods, insects included, are essentially crustaceans that moved onto land.4PubMed Central. Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic
This means crabs and lobsters are more closely related to butterflies and beetles than they are to, say, spiders. It also means the common kitchen question of whether a lobster is “basically a big bug” has a germ of evolutionary truth behind it, even though no biologist would classify a lobster as an insect. Both belong to Pancrustacea, but insects occupy a specific nested position within that larger group.
Not All Hexapods Are Insects
Here is where the nesting gets one layer finer. Hexapoda, the six-legged clade, includes insects but also contains three smaller groups that are not classified as insects: springtails (Collembola), two-pronged bristletails (Diplura), and coneheads (Protura). These tiny, often soil-dwelling creatures share the six-leg body plan with insects but differ in key anatomical details, such as mouthpart structure and the way their legs attach to the body. Phylogenetic reconstructions using ribosomal protein genes support the idea that Hexapoda as a whole is a genuine evolutionary group, not an artificial assemblage.5BMC Evolutionary Biology. Revealing pancrustacean relationships: phylogenetic analysis of ribosomal protein genes places Collembola (springtails) in a monophyletic Hexapoda and reinforces the discrepancy between mitochondrial and nuclear DNA markers
So the hierarchy runs like this: Arthropoda contains Pancrustacea, which contains Hexapoda, which contains Insecta. Every insect is a hexapod, every hexapod is a pancrustacean, and every pancrustacean is an arthropod. But at each level, there are members that do not belong to the smaller nested group. A springtail is a hexapod but not an insect. A barnacle is a pancrustacean but not a hexapod. A spider is an arthropod but not a pancrustacean.
Why Spiders, Ticks, and Scorpions Are Not Insects
One of the most common mix-ups people make is calling spiders insects. Spiders are arthropods, yes, but they belong to the chelicerates, an entirely separate major branch. The split between chelicerates and the lineage leading to insects happened deep in the Cambrian period, over 500 million years ago. Chelicerates have eight legs (or more), lack antennae, and have a body divided into two main sections rather than the insect’s three. Scorpions, ticks, mites, and horseshoe crabs also fall in this group.
Misconceptions about which arthropods are insects run deep enough that entomologists have written about the challenge of correcting them in classrooms and public outreach. Researchers who work with non-specialist audiences note that students of all ages arrive with persistent misinformation about arthropod classification.6Oxford Academic. Debugging Misconceptions About Arthropods The confusion makes sense: in everyday language, “bug” covers everything from beetles to centipedes to spiders. Biologically, those three animals sit on completely different branches of the arthropod tree.
Myriapods cause similar confusion. Centipedes and millipedes are sometimes casually called insects, but they belong to their own major group. Myriapods were among the first arthropods to colonize land, doing so during the Silurian period roughly 400 million years ago in an event independent from the terrestrial invasions of arachnids and insects.7PLoS Biology. Millipede genomes reveal unique adaptations during myriapod evolution Today there are about 16,000 described myriapod species, all terrestrial.
How Deep the Split Goes
Arthropods are ancient. Molecular clock estimates suggest that the split between millipedes and centipedes happened around 510 million years ago, in the late Cambrian. Arachnid lineages began diversifying around 472 million years ago, and hexapod radiations started around 483 million years ago, both in the Early Ordovician.8Current Biology. Molecular Timetrees Reveal a Cambrian Colonization of Land and a New Scenario for Ecdysozoan Evolution Other molecular analyses place the divergence of insects from their closest crustacean relatives near the Silurian-Ordovician boundary, roughly 420 to 434 million years ago, around the same time the first large land plants appeared.9PubMed. An insect molecular clock dates the origin of the insects and accords with palaeontological and biogeographic landmarks
Those timescales put the insect-spider split at over half a billion years. For comparison, the last common ancestor of humans and mice lived roughly 80 to 90 million years ago. The gulf between a beetle and a spider is far wider, in evolutionary terms, than the gulf between you and a mouse. When someone asks “is a spider basically an insect?” the honest answer is that the two groups are about as distantly related as two arthropods can be while still both being arthropods.
Arthropods That Sit Just Outside the Phylum
Right at the boundary of Arthropoda are two groups that share some arthropod traits but are classified separately: velvet worms (Onychophora) and tardigrades (Tardigrada). Together with arthropods, they form a larger grouping called Panarthropoda. Velvet worms have soft, unsegmented bodies and stubby unjointed legs, but they share a common ancestor with arthropods and have been found to possess a microRNA (miR-305) in common with arthropods that tardigrades lack.10PubMed Central. MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda Tardigrades, the famously near-indestructible “water bears,” sit one step further out. All three panarthropod groups share a different microRNA (miR-276) not found in other animal phyla, which confirms their shared ancestry even though velvet worms and tardigrades lack the defining arthropod features of a rigid exoskeleton and jointed legs.
These borderline relatives help illustrate what “arthropod” actually means in practice. If you have a hard exoskeleton, jointed appendages, and you molt, you are in. Velvet worms molt but lack the rigid exoskeleton and true joints, so they stay outside the phylum. Tardigrades molt too, but their body plan is so different that they have their own separate phylum. The boundaries are drawn by a combination of anatomy and shared evolutionary history, and insects fall well inside those boundaries.
Why Insects Dominate the Arthropod Count
Arthropoda is the most species-rich phylum on Earth, and insects account for the overwhelming majority of that richness. Current estimates put the number of described insect species above one million, with many more awaiting formal description. Insects are often considered the most successful group of animals in the terrestrial environment, represented by enormous biomass and outsized effects on ecosystems.11PubMed. An insect-specific system for terrestrialization: Laccase-mediated cuticle formation Part of that success comes from flight, which evolved in insects and in no other arthropod lineage. Winged insects could colonize new habitats and escape predators in ways unavailable to their ground-bound or aquatic relatives.
But the other arthropod groups are no slouches in diversity. Arachnids include roughly 100,000 described species, crustaceans around 70,000, and myriapods about 16,000. The total arthropod count dwarfs that of any other animal phylum. So while the answer to “are all insects arthropods?” is a clean yes, it is worth appreciating that insects are just the most conspicuous residents of an extremely large house.
A Shared Genetic Toolkit With Wildly Different Results
One of the more surprising findings from comparative genomics is how similar the underlying genetic toolkit is across all arthropods, despite the dizzying range of body forms. Surveys of Hox genes, the master regulators that tell developing embryos which body parts go where, show that extensive duplication and diversification of these genes happened long before the appearance of the visible differences in arthropod body plans.12Current Biology. Arthropod evolution: Same Hox genes, different body plans In other words, a beetle, a spider, and a shrimp all deploy variations of the same ancient set of body-patterning genes. The differences in how many legs they have, whether they grow wings, and how their heads are organized come down largely to when and where those shared genes are switched on during development.
The hormonal machinery for molting shows a similar pattern: genes involved in producing ecdysteroids have been found across all investigated arthropod genomes, from insects to chelicerates. There are some differences in the details. Chelicerates appear to use a slightly different form of molting hormone than insects do, probably because they lack one of the enzymes in the typical pathway.13Genome Biology and Evolution. How Did Arthropod Sesquiterpenoids and Ecdysteroids Arise? Comparison of Hormonal Pathway Genes in Noninsect Arthropod Genomes But the broad architecture is conserved. The common ancestor of all living arthropods already had the basic genetic equipment for building an exoskeleton, molting out of it, and patterning a segmented body. Its descendants just turned the dials differently.
When Everyday Language Trips People Up
Much of the confusion around insects and arthropods comes from the gap between common English and biological classification. “Bug” has no strict scientific meaning outside of true bugs (order Hemiptera). “Creepy-crawly” covers everything with too many legs. Even “insect” gets applied loosely to anything small that scuttles. Meanwhile, “crustacean” sounds like it should only mean things with claws that live in the ocean, even though woodlice (pill bugs) are crustaceans that live under logs in your garden.
The practical takeaway is straightforward. If someone asks whether a particular creature is an insect, count the legs and look at the body. Six legs and three body sections (head, thorax, abdomen) are the insect hallmark. Eight legs and two body sections point toward an arachnid. Lots of legs and an elongated body suggest a myriapod. A hard shell, multiple leg pairs, and a fondness for water usually mean crustacean, though terrestrial crustaceans like woodlice break that rule. All of these are arthropods. Only the six-legged, three-part creatures are insects, and even then, a handful of six-legged hexapods like springtails fall outside the insect class. The question “are all insects arthropods?” has a simple answer. The question “is this particular arthropod an insect?” is where things get interesting.